Meaning for the purpose of or in order to: Para bailar la bamba, necesita una poca de gracia. In order to dance the bamba you need a little grace.

With a noun or pronoun as object, meaning for the benefit of or directed to: Es para usted. It's for you.

Meaning to or in the direction of when referring to a specific place: Voy para Europa. I'm heading to Europe.

Meaning by or for when referring to a specific time: Necesito el regalo para mañana. I need the gift for tomorrow. Vamos a la casa de mi madre para el fin de semana. We're going to my mother's for the weekend.

Meaning for the purpose of or in order to: Para bailar la bamba, se necesita una poca de gracia. In order to dance the bamba you need a little grace.

With a noun or pronoun as object, meaning for the benefit of or directed to: Es para usted. It's for you.

Meaning to or in the direction of when referring to a specific place: Voy para Europa. I'm heading to Europe.

A o para.

Meaning by or for when referring to a specific time: Necesito el regalo para mañana. I need the gift for tomorrow. Vamos a la casa de mi madre para el fin de semana. We're going to my mother's for the weekend.

- "Por avión" used to be a literal translation of French "par avion", to indicate that ordinary mail should be sent by plane, so sending something via air transport is the only case I can think of when some people would use "por avión" instead of "en avión".

- However, I have to disagree about the case of "me caí por la nieve".
"Por" can be actually used to give a reference about the place where something has happened, but not exactly the place:
· Me caí por el faro. (I fell somewhere nearby the lighthouse.)
· Tuvimos el accidente por Mérida. (We had the accident somewhere nearby Mérida.)
In these cases, "por" is not a cause, but a place around which something has happened. There is no immediate ambiguity in the meaning, because one wouldn't expect a lighthouse to be the cause of someone falling down, or a city the cause of someone having an accident.
In the case of the snow, I wouldn't find any ambigüity either, because falling somewhere around or nearby the snow would sound strange to me (as if it would be concentrated only in one spot).
We would say "me caí en la nieve" to say "I fell on the snow".

And by the way, here it's correct to say "voy a casa de mi madre para el fin de semana". Although speakers might not be fully aware of it, it's an ellipsis that's felt as saying "...para pasar el fin de semana".
Still, it's colloquial and the right expressions are those Irma suggested.

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